1. Field of the Invention
This invention relates to an ignition system for an internal combustion engine, and more particularly to such an ignition system which is adapted to control a primary current of an ignition coil by means of a semiconductor switch to induce high voltage for ignition.
2. Description of the Prior Art
As an ignition system of such type which has been conventionally proposed in the art, an electronic control ignition system is known which is constructed to obtain ignition position advance characteristics using an electronic circuit comprising integrating circuits, a comparator circuit and the like. Such a conventional ignition system typically includes first and second signal coils which are adapted to generate first and second signals at the maximum advanced position and minimum advanced position of an engine, respectively. A flip flop circuit is driven by means of the first and second signals from the signal coils, to thereby generate an ignition operation allowable interval detecting signal (hereinafter referred to as "allowable interval detecting signal") continuing over an ignition operation allowable interval (hereinafter referred to as "allowable interval") corresponding to the interval from the maximum advanced position to the minimum advanced position and an ignition operation prohibiting interval detecting signal (hereinafter referred to as "prohibiting interval detecting signal") continuing over an ignition operation prohibiting interval (hereinafter referred to as "prohibiting interval") corresponding to the interval from the minimum advanced position to the subsequent maximum advanced position. First and second integrating circuits are controlled by such detecting signals to respectively generate a first integrating voltage having a waveform in which a voltage value is initially kept constant during the prohibiting interval from the minimum advanced position to the next maximum advanced position, then linearly increased during the allowable interval from the maximum advanced position to the next minimum advanced position and finally returned to the initial value at the minimum advanced position, and a second integrating voltage having a waveform in which a voltage value is increased at a constant gradient during the prohibiting interval from the minimum advanced position to the next maximum advanced position, then kept constant during the allowable interval between the maximum advanced position to the next minimum advanced position and finally returned to zero at the minimum advanced position, respectively. The so-generated first and second integrating voltages are compared in a comparator in order to detect a time when the first integrating voltage is larger than the second one. Upon the detection, a primary current control switch is turn off to carry out the ignition operation.
The electronic control ignition system of such construction as described above has an advantage capable of precisely determining an advanced width, an advance initiating engine speed and an advance terminating engine speed. However, it must use, for the purpose of obtaining the allowable interval detecting signal and prohibiting interval detecting signal, a flip flop circuit in which a number of logic elements including NAND circuits and the like are combined together. This causes the ignition system to be significantly large-sized because the arrangement of such flip flop circuits in the system requires a large space. Also, the conventional ignition system has another disadvantage that the trigger level of the logic elements is low sufficiently to readily cause the malfunction of the logic elements due to noise, leading to the malfunction of the flip flop circuit due to noise occurring at the generation of ignition spark. The conventional ignition system exhibits a further disadvantage that when a transistor is used as a switch element for controlling the charging of a first integrating capacitor, the first integrating capacitor is varied in charging voltage depending upon h.sub.FE of the transistor, so that the advance initiating engine speed is varied due to an ambient temperature.
In an ignition system of the current interrupting type that the ignition operation is carried out by turning off a primary current control switch at the ignition timing of an engine, it is required to lengthen to a certain degree a time during which the primary current control switch is kept at a turned-off state in order to lengthen a period of time during which ignition spark is generated to exhibit satisfied ignition performance. However, in a conventional ignition system of such type, a first integrating capacitor is often driven or triggered due to noise generated by spark discharge immediately after the spark discharge occurs at an ignition plug. The occurrence of such a phenomenon causes a first integrating voltage to be equal to below a second integrating voltage immediately after the spark discharge occurs at the ignition plug, so that a period of time during which a comparator keeps the primary current control switch at a turned-off state is substantially shortened to deteriorate the ignition performance.